An offbeat setting, this one : a world in perpetual night, save for an artificial sun; an Edwardian style setting, with occasional anachronisms like the one motorbike shown; D&D style wilderness between each settlement, inhabited by mechanical steam-punk style bugs (gaichuu), and the main traffic between towns are the Letter Bees (equally often referred to as such in English, as well as Tegami Bachi in Japanese. And there's a lot of crazy Engrish in the naming of places (Broccoli Forest, Breath Mint Pass,...).
The story opens with one Bee, Gauche Suede, finding a young boy, Lag Seeing, who has been marked with enough postage to take him to his aunt for fostering -- this serves to introduce us to the setting, the postmen Bees with the magic amber powered guns (the setting collectively being known as Amberground) that enable them to channel their energies into effective attacks against the gaichuu, and their companions, known as dingos; then timeskip and the boy now wants to follow in his rescuer's career.
On the way to his interview, he happens across a girl in similar circumstances to his own who has been abandoned with insufficient postage -- so to prove himself he undertakes the delivery, only to find she's being sent to a freak show; outcome, the half-monster girl he's named Niche (after where he found her) becomes his dingo.
With Lag, we find out more about this world; and we regularly get sentimental tales about the feelings bound into letters -- the whole series is really a paean to the old-fashioned snail-mail correspondence of yore -- especially as Lag's magical energies are just designed to evoke these sorts of echoes from anything they strike.
And there's a long running thread behind all of this about what happened to Gauche after he just went missing some point during the timeskip, the presence of rebels against the settled order of things, and what was going on when the artificial Sun went out for a few minutes the day Lag was born, and a government airship came crashing down? There's a second season planned, so no doubt we'll find out then.
Unusually, the secondary characters include a wheelchair-bound girl, Suzette, who is Gauche's younger sister and shares birth date with Lag. The question of how she navigates doorsteps as tall as the radius of her main wheels is finessed -- in fact there's a distinct lack of adaptations in her house.
One thing that struck me about this series was the background music -- not just Sub-Postmistress Aria playing Air on a G string (lower right); but all though, there was a good string-based orchestral sound track. OP and ED were J-pop, with, to my mind, the first pair better than the ones that took over half way through.